One prior art method of joining sheets has been to apply a layer of heat-fusible adhesive either between the layers or along the edge of the overlap.
Another method of welding thermoplastic materials is to use frictional heat generated at the point of weld by high frequency mechanical motion. This has been done by rubbing a tool moving at high frequency and small amplitude against the thermoplastic material at the point of weld, to generate frictional heat, such as is shown in Balamuth U.S. Pat No. 3,321,558.
Another technique is shown in Balamuth U.S. Pat. No. 3,376,179. There, a single vibrating tool rubbing against the workpiece itself was used to generate heat. The tool was placed in the nip where two sheets of material came together and vibrated at high frequency, the objective being to generate sufficient heat to join the sheets by fusion. His tool, however, reciprocated in a direction parallel to the nip between the materials and, for reasons set forth below in connection with the discussion of FIGS. 7 and 8, would not produce a satisfactory uniform seal. As a result, it has not been commercially successful.
By contrast, the present invention uses an ultrasonic transducer to vibrate a welding horn and positions the horn so that the horn vibrates against the two sheets being welded in a direction perpendicular to the nip. This relationship produces a uniform seal because the uniformity of the motion of the tip of the horn serves to create the same amount of frictional heat at all points in the nip. For wide horns, the uniformity of motion can be further enhanced by using structures such as those shown in Holze U.S. Pat. No. 4,363,992.